This invention relates to a chain tensioner for keeping constant the tension of a timing chain for driving camshafts in an automotive engine.
In an automotive engine, the rotation of the crankshaft is transmitted to camshafts through a timing chain to rotate the camshafts, thereby repeatedly opening and closing valves of combustion chambers. In such an engine, in order to keep the tension of the chain within a proper range, a tension adjusting device is frequently used which comprises a chain guide pivotable about a shaft, and a chain tensioner biasing the chain guide in the direction to press the chain guide against the chain.
Such chain tensioners typically comprise a cylinder having a closed end, a plunger axially slidably inserted in the cylinder for pressing a chain. The plunger defines in the cylinder a pressure chamber filled with hydraulic oil. The volume of the pressure chamber is variable with the axial movement of the plunger. The plunger is biased by a spring in the direction to increase the volume of the pressure chamber. The cylinder is formed with an oil supply passage through which the pressure chamber communicates with the outside of the cylinder. The oil supply passage is provided with a check valve which allows only a flow of hydraulic oil through the oil supply passage from the outside of the cylinder to the pressure chamber (JP Patent Publication 2006-17214).
In this chain tensioner, when the tension of the chain decreases, the plunger is pushed out of the cylinder by the spring, thereby re-tensioning the chain. When the plunger moves in this direction, the hydraulic oil is supplied into the pressure chamber through the oil supply passage, so that the plunger moves quickly, thus quickly re-tensioning the chain.
When the tension of the chain increases, the plunger retracts under the tension of the chain, thereby absorbing the tension of the chain. When the plunger retracts, the check valve closes, and hydraulic oil in the pressure chamber leaks through a restricted passage defined between the sliding surfaces of the plunger and the cylinder to the outside of the cylinder. The plunger can thus retract slowly, so that the tension of the chain stabilizes.
The check valve comprises an annular valve seat, a check ball which is movable into and out of contact with the valve seat, and a retainer retaining the check ball and comprising a tubular portion and a bottom at one end of the tubular portion. The tubular portion of the retainer is formed with an opening through which hydraulic oil can flow into and out of the retainer. The opening extends to a border between the tubular portion and the bottom of the retainer. The retainer is formed by drawing a circular blank having the opening formed by punching.
While the engine is running, fluctuations in torque of the crankshaft and rotational resistance of the camshafts are transmitted to the chain, so that the pressure in the pressure chamber fluctuates with high frequency. In the case of a four-cylinder engine, torque fluctuates twice per rotation of the crankshaft. Thus, while this engine is rotating at 6000 rpm, the pressure in the pressure chamber fluctuates with a frequency of 200 Hz. When the pressure in the pressure chamber fluctuates with such high frequency, the check ball of the check valve repeatedly collide against the bottom of the retainer, thus producing stress at the bent portion of the retainer, i.e. the border between the bottom and the tubular portion of the retainer. At this time, the radially inner portion of the border is subjected to higher stress than is the radially outer portion of the border because the former has a smaller radius of curvature than the latter.
FIG. 9 shows the abovementioned conventional retainer 51. As shown, the opening 52 in the tubular portion 51A of the retainer 51 is formed by punching the tubular portion from outside to inside thereof. Thus, as shown in FIG. 10, the punched section formed by punching comprises a shear surface 53 and a ruptured surface 54 which is rougher than the shear surface 53 and located radially inwardly of the shear surface 53. Thus, when the check ball (not shown) collides against the bottom 51B of the retainer, the ruptured surface 54 tends to be subjected to higher stress than the shear surface 53. Since the ruptured surface 54 tends to be subjected to higher stress and is rougher than the shear surface 53, cracks tend to develop in the ruptured surface 54 due to stress concentration, which can result in fatigue failure of the retainer 51.
An object of the present invention is to improve the durability of the retainer of the check valve, thereby improving the reliability of the chain tensioner.